The present invention is generally related to a controller for a radiotelephone, and more particularly to a radiotelphone microcontroller having a secure mode of operation and a nonsecure mode of operation, each mode sharing common portions of a microcontroller while retaining independent secure and nonsecure functions.
Previously, operation in two separate and functionally distinct modes--providing secure operation for one application and nonsecure operation for another--has been accomplished using several microcontrollers. Each microcontroller would operate in a wholly independent fashion, employing separate hardware while operating simultaneously beginning at power up. As a particular example, a mobile radiotelephone system such as that employed in the Nordic countries (and generally known as the Nordic Mobile Telephone, or NMT) and implementing SIS (Subscriber Identity Security--which is further defined in SIS Addendum to NMT DOC 900-3, dated July 1, 1988) has required three microcontrollers. The first microcontroller, the main system microcontroller, has been responsible for the radiotelephone system process execution. The second microcontroller, the display microcontroller, has been responsible for both display and keypad functions and interface with the radiotelephone user. The third microcontroller, the SIS microcontroller, has been responsible for the security of the SAK (Subscriber Authentication Key, the telephone call authorization number which is further described in SIS Addendum to NMT DOC 900-3 dated July 1, 1988 and as defined by each of the Nordic countries) and intermediate calculations required in the use of the SAK. The SAK itself is a random and user-specific 120 bit binary word which, among other parameters, is programmed into the radiotelephone unit at the point at which the user takes possession of the radiotelephone equipment. It is used when a radiotelephone call is to be made to ensure that the radiotelephone equipment is authorized to make a call. Typically, the SAK is not used in receiving a call.
In miniature portable radiotelephones (such as the MICROTAC portable radiotelephone available from Motorola, Inc. as model number F09EFD6930AA) physical space is at a premium and techniques of reducing the size of the electronic circuitry are required to achieve the necessary size reductions. Merely increasing the processing capacity of one microcontroller and loading both the nonsecure and the secure operating modes into the one microcontroller does not protect the secure operation mode from undesired external access. Furthermore, since the secure operation mode includes not only the storage and handling of the SAK but also the calculations and intermediate results from the calculation, both the storage and the calculations must be protected from access.